Rewrite images tests with mock

test_images module was using fixtures to test ironic.common.images
module. Mock is now used instead. New tests added. fetch_to_raw is
not tested as it is going to be removed as mentioned here:
https://review.openstack.org/#/c/115974/

Change-Id: Id931b6d9cc1213f1347a9324510580e14c601412
Closes-bug: #1361672
Merge "Rewrite images tests with mock"